Nanotechnology, materials science, and life sciences demand the ability to form images at nanometer scale resolution. For example, integrated circuits are now manufactured with features as small as a few tens of nanometers, and the development and control of integrated circuit manufacturing processes requires forming useful images of such feature. Variations in the lithography processes used to make integrated circuit necessitate continually monitoring or measuring the process results to ensure that the product parameters remain within acceptable ranges.
The importance of such monitoring increases considerably as minimum feature sizes approach the limits of resolution of the lithographic process. Features to be monitored may include the width and spacing of interconnecting lines, spacing and diameter of contact holes, and the surface geometry such as corners and edges of various features. Features on a semiconductor wafer are three-dimensional structures and a complete characterization must describe not just a surface dimension, such as the top width of a line or trench, but a three-dimensional profile of the feature. It is also necessary analyze contamination and other defects that are found in the fabrication process.
Some observations and measurements can be made using a scanning electron microscope (SEM). In an SEM, a primary electron beam is focused to a fine spot that scans the surface to be observed. Secondary electrons are emitted from the surface as it is impacted by the primary beam. The secondary electrons are detected, and an image is formed, with the brightness at each point of the image being determined by the number of secondary electrons detected when the beam impacts a corresponding spot on the surface. As features to be observed continue to get smaller and smaller, however, there comes a point where the features to be measured are too small for the resolution provided by an ordinary SEM.
Transmission electron microscopes (TEMs) allow observers to see extremely small features, on the order of nanometers. In contrast to SEMs, which only image the surface of a material, TEMs also allows analysis of the internal structure of a sample. In a conventional TEM, a broad beam impacts the sample that is held in a holder referred to as a “TEM grid” and electrons that are transmitted through the sample are focused to form an image. The sample must be sufficiently thin to allow many of the electrons in the primary beam to travel though the sample and exit on the opposite site. TEM samples are typically less than 100 nm thick.
In a scanning transmission electron microscope (STEM), a primary electron beam is focused to a fine spot, and the spot is scanned across the sample surface. Electrons that are transmitted through the work piece are collected by an electron detector on the far side of the sample, and the intensity of each point on the image corresponds to the number of electrons collected as the primary beam impacts a corresponding point on the surface.
Because a sample must be very thin for viewing with transmission electron microscopy (whether TEM or STEM), preparation of the sample can be delicate, time-consuming work. The term “TEM” as used herein refers to a TEM or an STEM, and references to preparing a sample for a TEM are to be understood to also include preparing a sample for viewing on an STEM.
The term “substrate” is used herein to refer to the work piece from which the sample is extracted, and the term “sample” will be used to describe the portion of the substrate that is extracted from the substrate and mounted onto a TEM grid for thinning and/or for observation.
Several techniques are used for preparing TEM specimens. These techniques may involve cleaving, chemical polishing, mechanical polishing, or broad beam, low energy ion milling, or combining one or more of the above. The disadvantage to these techniques is that they often require that the starting material be sectioned into smaller and smaller pieces, thereby destroying much of the original work piece.
Other techniques, generally referred to as “lift-out” techniques, use a focused ion beam to cut the sample from a substrate or bulk sample so that it can be lifted out without destroying or damaging surrounding parts of the substrate. Such techniques are useful in analyzing the results of processes used in the fabrication of integrated circuits, as well as in analyzing materials in the physical or biological sciences. These techniques can be used to form samples from any orientation within the substrate (e.g., either in cross-section or in plan view). Some lift-out techniques extract a sample in the form of a lamella sufficiently thin for use directly in a TEM; other lift-out techniques extract a “chunk” or large sample that requires additional thinning before observation. The sample may be thinned while still attached to the substrate, while attached to a probe used to transport the sample from the substrate to the TEM grid, or after it is attached to the TEM grid. The lamella may be formed as a uniformly thin structure or it may include a thin viewing area within a thicker support structure. An extracted lamella typically forms a sample oriented normal to the substrate surface. A chunk is often extracted to form a sample parallel to the substrate surface as described in U.S. Pat. No. 7,423,263 to Hong et al. for “Planar View Sample Preparation,” which is owned by the applicant of the present invention and which is hereby incorporated by reference.
Techniques in which the prepared sample is extracted from the substrate and moved to a TEM grid within the focused ion beam (FIB) system vacuum chamber are commonly referred to as “in-situ” techniques. Techniques in which the sample is formed by a focused ion beam and then the substrate is removed from the vacuum chamber before the sample is removed from the substrate are call “ex-situ” techniques.
In one technique, samples are thinned to the desired thickness before they are separated from the substrate and the samples are transferred to a metallic grid covered with a thin electron transparent film. The sample is viewed by passing an electron beam through the sample as it rests on the film. FIG. 1 shows a prior art TEM grid 100, which is typically made of copper, nickel, or gold. Although dimensions can vary, a typical grid might have, for example, a diameter of 3.05 mm and have a middle portion 102 consisting of cells 104 of size 90 μm×90 μm defined by bars 106 which have a width of 35 μm. The electrons in an impinging electron beam will be able to pass through the cells 104, but will be blocked by the bars 106. The middle portion 102 is surrounded by an edge portion 108. The width of the edge portion 108 is 0.225 mm. The edge portion 108 has no cells and displays an orientation mark 110. The electron transparent support film is approximately 20 nm thick and uniform across the entire sample carrier. TEM specimens to be analyzed are placed or mounted within cells 104.
To remove the sample from the substrate, a probe attached to the micromanipulator is positioned over the sample and carefully lowered to contact it. For ex-situ removal, the probe can use a vacuum, electrostatic forces, or an adhesive to attach the sample to the probe tip to move it from the substrate to the grid. One such system for ex-situ extraction of samples is described in U.S. Pat. No. 8,357,913 to Agorio et al. for “Method and Apparatus for Sample Extraction and Handling.”
Rather than thinning the sample before it is removed from the substrate, in some in-situ processes samples are removed from the substrate using a probe connected to a micromanipulator and attached to a post (also referred to as a “tooth” or “finger”) of a TEM grid such as the one shown in FIG. 2. The partly or fully formed sample is typically attached to a probe by beam-induced deposition after it is formed. The sample is then separated from the substrate and transported by the probe to a TEM grid, where is it attached by beam-induced deposition to a post. The connection between the probe and the sample is then severed, leaving the sample on the TEM grid post. The sample probe may be rotated and the TEM grid may be tilted or rotated to ensure that the sample is attached to the post in the desired orientation for processing an viewing. Techniques for forming and extracting samples are described, for example, in U.S. Pat. Pub. No. 2013/0248354 for “High Throughput TEM Preparation processes and Hardware for Backside Thinning of Cross-Sectional View Lamella” by Keady et al. and in WO2012/103534 for “TEM Sample Preparation” of Blackwood et al., both of which are owned by the applicant of the present application and are hereby incorporated by reference.
A typical post-type TEM grid 200 comprises a portion of a 3 mm circle. In some applications, samples, such as sample 202A, 202B, and 202C, are attached to posts 204A, 204B, 204C, or 204D of the TEM grid 200 by ion beam deposition or an adhesive. The sample extends from the post so that an electron beam in a TEM (not shown) will have a free path through the sample to a detector under the sample. The sample is typically mounted with the thin viewing area parallel to the plane of the TEM grid, and the TEM grid is mounted so that the plane of the TEM grid is perpendicular to the electron beam when the sample is observed. FIG. 3 shows another TEM grid 300 having posts 302A and 302B to which samples 304A, 304B, 304C, and 304D are attached. The posts of TEM grid 300 have a different shape than the posts of TEM grid 200.
FIGS. 5A-5C show enlarged views of a post 500 of a typical prior art TEM grid. The TEM post typically includes a shelf 502 referred to as a “setback.” FIG. 5A illustrates a front-view of the post 500 showing the shelf 502 formed by an outer edge 504 that is thinner than the interior portion 506 of post 500. The setback facilitates attachment of the sample 508 to the post 500 and minimizes or avoids damage to the TEM grid during attachment. The shelf 502 may run continuously around the perimeter of the entire TEM post. FIG. 5B illustrates a side-view of the post 500 FIG. 5A. FIG. 5C is a photomicrograph showing a TEM post 500 having a shelf 502. The thinner outer edge 504 extends along the entire post 500 and between posts.
Although, the setback is not shown in FIGS. 2, 3, 4A, 4B, 8A, and 8B, it should be recognized that the TEM grids of each have a setback on which the sample or lamella is attached to. The setback is not shown for clarity purposes in these other figures.
The term TEM grid is used herein to refer to any structure onto which the sample is mounted including not only a metallic grid covered with a thin electron transparent film as shown in FIG. 1, but also post-type grids as shown in FIGS. 2 and 3, as well as any other type of support, such as a wire to which samples can be attached. The TEM grid is typically mounted in a TEM sample holder. The sample holder can be removed from the ion beam system vacuum chamber to transport the TEM grid with the samples to a TEM for viewing. Sample holders for holding TEM grids and systems for transporting sample holders are known.
Preparation of TEM samples using prior art methods of sample extraction are time consuming. Critical Dimension (“CD”) metrology and other process monitoring techniques often requires multiple samples from different locations on a wafer to sufficiently characterize and qualify a specific process. In some circumstances, for example, it will be desirable to analyze from 15 to 50 TEM samples from a given wafer. When so many samples must be extracted and measured, the total time to process the samples from one wafer can be days or even weeks. Even though the information that can be discovered by TEM analysis can be very valuable, the entire process of creating and measuring TEM samples has historically been so labor intensive and time consuming that it has not been practical to use this type of analysis for manufacturing process control.
Speeding up the process of sample extraction and transfer would provide significant time savings by allowing a semiconductor wafer to be more rapidly returned to the production line. Automating the lift out process will increase the number of samples extracted by the ion beam system in a given time period.